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Neptuniumwas the first synthetic transuranium element of the actinide series discovered; the isotope neptunium-239 was produced by McMillan and Abelson in 1940 atBerkeley, California, as the result of bombarding uranium with cyclotron-produced neutrons. The isotope neptunium-237 (half-life of 2.14 X 10^6 years) iscurrently obtained in gram quantities as a by-product from nuclear reactors in the production of plutonium. Twenty isotopes and isomers of neptuniumare now recognized. Trace quantities of the element are actually found in nature due to transmutation reactions in uranium ores produced by the neutronswhich are present. Neptunium is prepared by the reduction of Npf3 with barium or lithium vapor at about 1200°C. Neptunium metal has a silveryappearance, is chemically reactive, and exists in at least three structural modifications: a-neptunium, orthorhombic, density 20.25 g/cm^3, beta-neptunium(above 280°C), tetragonal, density (313°C) 19.36 g/cm^3; g-neptunium (above 577°C), cubic, density (600°C) 18.0 g/cm^3. Neptunium has four ionicoxidation states in solution: Np+3 (pale purple), analogous to the rare earth ion Pm+3, Np+4 (yellow green); NpO+ (green blue); and NpO++ (pale pink).These latter oxygenated species are in contrast to the rare earths which exhibit only simple ions of the (II), (III), and (IV) oxidation states in aqueoussolution. The element forms tri- and tetrahalides such as Npf3, Npf4, NpCl4, NpBr3, NpI3, and oxides of various compositions such as are found inthe uranium-oxygen system, including Np3O8 and NpO2. Fifteen isotopes of neptunium are now recognized. The O.R.N.L. has neptunium-237 available for saleto its licensees and for export. This isotope can be used as a component in neutron detection instruments. It is offered at a price of $660/g plus packingcosts. 1

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